The present invention relates to an anode driving apparatus of a rotating anode X-ray tube.
In general, an X-ray tube used in an X-ray CT apparatus must radiate X-rays continuously while it is rotating around a subject being examined. Therefore, if electrons from a cathode are continuously radiated only onto a single spot on the target of an anode, the target will eventually melt. For this reason, the anode is provided with an inclined target and is rotated at a constant speed, thereby preventing radiation of electrons from the cathode solely onto a single spot on the target.
FIG. 1 shows a conventional driving apparatus for a rotating anode of such an X-ray tube. In FIG. 1, secondary winding T (output side) of a power transformer is provided with three taps, three voltages (AC voltages)--of 220 V, 110 V, and 60 V--being output therefrom, respectively. The voltage of 220 V is used during activation, wherein the anode in a static state is made to begin rotating, the voltage of 60 V is a driving voltage used to keep the anode rotating at a constant speed, and the voltage of 110 V is a braking voltage used to stop the rotation of the anode. The driving voltages of 220 V and 60 V are mechanically switched by a switching circuit constituted by three electromagnetic relays Ry1, Ry2, and Ry3, in accordance with a selected one of two modes, i.e., an activation mode and a constant-speed rotation mode, and are applied to common terminal C of an AC motor (not shown) for rotating the anode. The braking voltage of 110 V is rectified through a full-wave rectifier consisting of diode bridge circuit STACK 1 and is then supplied to main terminal M of the AC motor via two relays, Ry2 and Ry3, and to subterminal S further via phase-shift capacitor C1.
The switching of relays Ry1 to Ry3 is controlled in association with a rotate signal supplied from an external circuit (not shown), as is shown in FIG. 2. In this case, assume that each electromagnetic relay is in an ON state and its contact is connected to a terminal indicated by a white dot when the level of FIG. 2 is high level.
In this conventional apparatus, electromagnetic relays are used to switch the driving and braking voltages to be applied to the AC motor. Therefore, an arc is generated, which is turn generates a surge dependent on a voltage phase whenever the voltage is switched (i.e., since an AC voltage is used, an arc is not generated when a phase is 0 but is generated when the phase is other than 0). In this case, the electromagnetic relays become a large noise source with respect to other circuits. Moreover, each time a surge is generated, this causes damage to, and a gradual wearing away of the relay contract, with the result that the electromagnetic relays must be replaced periodically.
In addition, the braking voltage is rectified by a special rectifier separate from a circuit for the driving voltage. Since additional space must be provided to house the rectifier, this results in an apparatus which is quite bulky.